This is a Duroc.

Most popular pig breeds in the usa?

The most popular breeds of pigs in the United States are a
Hampshire  , a Poland  China, a Duroc, a Chester  White ,a Berkshire and a American
Yorkshire.

This is a Hampshire. They almost always are black with a pink or white stripe from one front leg onto the pigs back  and then to the other front leg.

This is a Berkshire . They are usually black and white.

This is a Poland China.  They are usually black with a little bit or a lot of white.

This is a  American Yorkshire.

This is a Chester White.

What are pigs main diet?

People who own pigs usually feed their pigs a balanced meal to
eat. It makes sure the pigs are healthy. Pigs need protein, carbohyydrates,
vitamins, and minerals. So the food that will match these nutrients are corn, 
wheat and  soybeans

Why do pigs roll in mud?

The truth about why pigs roll in mud and water is because pigs
don’t  have sweat glands. People 
sweat to  cool of but since pigs
don’t have sweat glands they have to roll in something  cool. Pigs can get
sunburned easily if they are light skinned. Like American Yorkshires which are pink. But if you have a dark colored pig like Durocs, Herefords and Hampshires. They won't get sunburned.

References

http://backyardranch.com/Pig_Breeds.htm
http://animals.pawnation.com/pigs-diet-3566.html  
www.whyzz.com/why-do-pigs-roll-around-in-the-mud‎